Folks,
Digital Cables part two....
We left below at:
3DSonics said:
BTW, in 1nS Light travels 12" (appx.) and most cables have less than 80%c propagation speed. In 100pS Light travles only about 1.2"....
Now, let us assume a S/P-DIF standard Digital cable, 1m long, 50% (of c) propagation speed (due to PVC insulation) and with RCA Plugs.
Sadly, no RCA sockets (except possibly WBT nextgen, but I'm not taking their word) have anywhere near 75 Ohm CI, many will be closer to 30 Ohm CI.
So a proportion of somewhere between 20 & 50% of the signal reaching the far end of the cable will be reflected. The round trip of this reflection back to the source (where another non-75R-CI connector reflects a good deal of the reflection) will take around 6 - 7nS so the now re-arriving reflection which still has an appreciable proportion of the signal strenth of the desired signal arrives well within the leading (and triggering) edge of the signal.
And let us remember, our signal will "trigger" the recevier (if it is a cirrus logic one) when a +/-250mV square wave exceeds 200mV.
The chances that the interference will effect the trigger window is not small, as with (say) 10nS rise time the triggering will be just around where the reflection arrives.
NOW, if we had an unchanging signal with a stable clock, that would matter FA, as the distrubances would be present on EACH AND EVERY transttion. But in S/P-DIF we change the clock to signifiy bits, so the disturbances on the triggering edge are signal dependent.
Now, if we make our cable sufficiently long that the "roundtrip" of relections take much longer than the triggering on the edge, but not so long as to interfere with the next triggering edge we will have eliminated this source of jitter (jtter due to interchassis differences, RFI etc remain).
Equally, if keep the cable very short (as I said, < 10..12" and ideally with a very high propagation velocity) we will not materially effect the edge triggering, as the reflection arrives almost instantanious back, a further advantage of very short cables is that other jitter soures (interchassis noise currents generating fault voltages in the non-zero impedance ground return, EMI, RFI et al) are also kept to an absolute minimum.
Anyway, that is more or less abstracted and simplified the reasoning behind "make digital S/P-DIF cables either VERY SHORT or comparably VERY LONG.
Of course, some listeners may prefer a system with more jitter and of a certain spectrum, which normal length digital cables will readily provide, but in a manner and fashion that remains extremely unpredictable from system to system and cable to cable.
The best solution is to avoid S/P-DIF entierly and the secondbest is to switch to AES/EBU connections as the XLR connectors (without metal shell) is pretty close to 100 Ohm CI and as the 5V Peak-Peak Level and balanced input to the receiver confine the triggering to a much narrower part of the leading edge of the signal. This still does not ensure that there are no other interactions of course, but it raises the bar aginst such things considerably (as do +20db @ full scale pro-style balanced connections do compared with 2V @ full scale consumer single ended connections).
Ciao T